Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controls2 = u *tRfor a surface with roughness elements (2.31) = 2su *tSfor a bare surfacewhere u *tR and u *tS are the threshold friction velocities for rough and smooth surfacesrespectively. The threshold friction velocity ratio (R t ) can then be computed as:u* tS ' s 1Rt = = =(2.32)u * tR( 1)( 1+ )where the factor (1 + βλ) -1/2 accounts for the reduction in surface stress by shelter fromroughness elements, and (1 - σλ) -1/2 accounts for the “amplification’ of the stress (τ’s) on theerodible surface over the difference between total stress and that acting on the roughnesselements (τ – τ R ) caused by the fraction of soil surface (σλ) occupied by the roughnesselements. This formulation was then modified to account for the fact that u *t is controlled bythe maximum stress (τ’’s) acting at any point on the surface, rather than spatially averagedstress (τ’s). The modification was thus designed to account for non-uniformity around nonerodibleroughness elements, and is implemented by the inclusion of the m parameter (wherem ≤ 1). Written for the computation of the threshold friction velocity over vegetated surfaces(u *tR ) as an adjustment to the threshold friction velocity of a smooth surface, we have:( 1m)( m )u* tR= u*tS1+(2.33)Key simplifications in the model are that: 1) surfaces are only described in terms of theroughness density, and the model does not account for differences in element shape; and 2)the model does not account for the presence of turbulent air flow close to the surface.Limitations identified in the drag partitioning scheme revolve around defining appropriatevalues for the m parameter, the applicability of the m parameter itself, and the roughnessdensity range for which the model is valid λ ≤ 0.1 (Okin and Gillette, 2004). The dragpartitioning scheme presented by Raupach (1992) and Raupach et al. (1993) was revised byYang and Shao (2005) to account for multiple drag partitioning (e.g. between roughnesslayers at different heights), and for roughness densities where λ ≥ 0.1.60